LONDON (Reuters) - Britain's government on Thursday published internal legal advice it had been given which it said showed it was legally entitled to take military action against Syria even if the United Nations Security Council blocked such action.

It also published intelligence material on last week's chemical weapons attack in Syria, saying there was no doubt that such an attack had taken place, that it was "highly likely" that the Syrian government had been behind it, and that there was "some" intelligence to suggest that was the case.

"If action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria," a copy of the British government's legal position read.

In such circumstances, it added that "military intervention to strike specific targets with the aim of deterring and disrupting further such attacks would be necessary and proportionate and therefore legally justifiable."

A letter from the chairman of Britain's Joint Intelligence Committee said there were "no plausible alternative scenarios" except that forces loyal to President Bashar al-Assad had perpetrated the chemical attack in the suburbs of Damascus.

"We also have a limited but growing body of intelligence which supports the judgment that the regime was responsible for the attacks and that they were conducted to help clear the Opposition from strategic parts of Damascus," it said.

(Reporting By Andrew Osborn and William James; Editing by Guy Faulconbridge)